1. Field of the Invention
The present invention relates to systems for controlling current in three phase AC induction motors. More particularly, the present invention relates to a system which uses zero crossing current information provided by a motor control system to determine if the currents in the separate stator windings of a three phase AC motor are unbalanced and, if so, rebalances the currents accordingly.
2. Description of the Art
One type of commonly designed induction motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source generating a current therein.
Modern power stations which supply voltage to AC motors usually supply well balanced three phase voltages having identical periods and amplitudes but having phases which differ by exactly 120 degrees. However, for various reasons, it is not uncommon for the voltages at the point of utilization, across the stator windings, to be unbalanced. Unbalanced stator voltages are generally recognized as undesirable for a plurality of reasons discussed below.
One major cause of unbalanced voltage is an electrical load which is not uniformly distributed on all three supply lines. Unbalanced loads frequently occur in rural electric power systems having supply lines of different lengths, but can also occur in larger urban power systems where there are heavy single-phase demands (e.g. heavy lighting loads on one of the three supply lines). Unbalanced voltage can also be caused by unsymmetrical transformer windings, unmatched transmission impedance or many other reasons.
Importantly, even a small voltage unbalance can result in a relatively large current unbalance in the stator windings of a three phase motor. For example, for a 3% voltage unbalance, the stator current unbalance might be 18% to 24%. This occurs because the voltage unbalance can be represented as a combination of a normal three phased positive sequence voltage component, plus a small negative sequence voltage component, which on its own would drive the motor in the reverse direction.
Unbalanced currents deliver uneven power to the rotor and thus produce undesirable torque pulsations and motor vibrations. The friction caused by the vibrating motor accelerates deterioration of the mechanical components of the motor. If the motor is operating at or near its fully rated load, the rotor and any stator windings carrying increased current heat up unnecessarily. While extreme overheating may trip an overload relay to switch off the motor and protect it from burning out, lesser degrees of unbalance usually go unchecked as the heat generated is insufficient to trip the overload relay.
Excess heat causes motor insulation to age at an accelerated rate and causes accelerated deterioration and evaporation of the bearing and other lubricant, both of which shorten the useful life of a motor. In addition, the excess motor heat is lost mechanical energy which means the motor is running inefficiently.
Control systems have been developed which can measure and correct current unbalance by regulating the period during which voltage is applied to the three stator windings. Many such control systems employ separate solid state switches connecting each stator winding to one of the three supply lines. Each solid state switch is formed by either a triac or a pair of back-to-back connected silicone controlled rectifiers (SCR's), commonly referred to as a thyristor.
The thyristor based control systems have a circuit which determines the proper time at which to trigger each thyristor switch during every half cycle of the associated supply line voltage. A triggered thyristor switch remains in a conductive state until the alternating current flowing through it goes to zero, after which time the thyristor must be triggered again to become conductive.
By altering the trigger times of the switches with respect to the zero crossings of the supply line current, the intervals during which the thyristors are conductive can be varied to control the amount of voltage applied to each stator winding and hence to control the current in each winding. Thus, rebalancing currents is relatively easy once unbalance is measured. However, measuring unbalance has generally been costly in both computational time and additional hardware.
Typically, current unbalance has been measured by placing transformers on each of the voltage supply lines. Each transformer isolates a current signal from the supply voltage and steps the current down from the high value found in motors (e.g. 10-1,000 A) to a value convenient to handle for signal processing (e.g. 100 mA). Deriving true RMS current values requires sampling the current in each line (at least 12 samples per cycle are needed, and usually 50 or more for RMS accuracy of a few percent). As rebalancing introduces harmonic frequency currents in each stator winding which mask fundamental frequency currents, filtering circuitry is usually needed to distill the fundamental frequency portion of each sample. Next, each fundamental frequency sample must be squared, the squares must be averaged to get a mean square value, and then the square root of the mean square must be calculated. Once the RMS current values for each cycle are known, the phase currents can be compared and rebalanced accordingly.
This solution to the current measurement problem requires special hardware in addition to that found in a typical motor controller. In addition, this solution requires a large amount of computational time which limits the ability of the motor controller to monitor other motor parameters. Therefore, it would be valuable to have a method by which current unbalance could be detected and corrected using information already supplied by the motor controller without direct measurement of current and without costly and time consuming current calculations.